Recently, there has been growing interest in energy storage technologies in the field of electrochemical devices. As the application fields of energy storage technologies have been extended to mobile phones, camcorders, lap-top computers and even electric cars, efforts have increasingly been made towards the research and development of batteries. In this aspect, electrochemical devices have attracted the most attention. The development of rechargeable secondary batteries has been the focus of particular interest.
Referring to FIG. 1, an exemplary embodiment of an electrode used in a conventional secondary battery includes an electrode 1 having an electrode active material layer 2, 2′ in which the electrode active material layer 2, 2′ is formed on both surfaces of a current collector 3 having a shape of a long sheet, and an active material-free portion (a) not covered with the electrode active material layer 2, 2′ is formed at an end portion of the current collector 3.
When an electrode assembly is formed by stacking an electrode and a separator and wound into a jelly roll, there may occur a phenomenon in which the separator is pressed by an edge of the electrode, to be exact, an edge of a portion in which electrode winding is completed, and as described in the foregoing, an electrode having an active-material free portion (a) has an advantage of alleviating or reducing a phenomenon, in which a separator is pressed by an edge of a portion where electrode winding is completed, due to the active-material free portion (a), and consequently relieving separator stress, thereby preventing a separator disconnection phenomenon from occurring.
However, in case the electrode 1 having the active material-free portion (a) is used, an active material slurry scatters over an active material-free portion of a current collector during coating of an electrode active material, in particular, a cathode active material, and as a result, an active material island is formed, and such an island may cause an internal short circuit of a battery when the current collector sheet stretches by the repetition of shrinkage and expansion of a jelly roll in a high capacity model life test or the like.
Accordingly, as proposed in FIG. 2, the free edge electrode 1 manufactured by forming the electrode active material layer 2, 2′ on both surfaces of the current collector 3 and cutting off to eliminate an active material-free portion is proposed.
In the specification, the term ‘free edge electrode’ is understood as representing an electrode in which both an active material layer and a current collector form an edge at the same location, as proposed in FIG. 2.
However, because the free edge electrode proposed in FIG. 2 avoids a problem of an island being formed by the scattering of an active material but raises a high density step issue due to the electrode active material layer and the current collector forming an edge at the same location, stress applied to the separator 5 by the edge of the electrode at a portion where winding of a stack of the electrode 1 and the separator 5 into a jelly roll is completed is even higher than stress applied when winding into a jelly roll using an electrode having an active material-free portion, and as a result, an issue with a separator being pressed and/or disconnected is exacerbated.